Vapor condensation system and method

ABSTRACT

A flowing film of a composition of water and a solute over a refrigerated surface, is at such solute concentration and freezing temperature as to de-sublimate or condense the water vapor as ice which is carried away by the film, and subsequently separated from the composition by filtering or centrifuging. A terminal portion of the refrigerated surface is shielded to prevent access of water vapor thereto to effect removal as ice of water that may be absorbed in the film from the vapor, thereby maintaining the film concentration substantially uniform. A salt solution or brine provides an advantageous solute.

United States Patent King, III et al.

[ Sept. 18, 1973 VAPOR CONDENSATION SYSTEM AND METHOD [75] Inventors: Cary Judson King, Ill, Kensington;

Romesh Kumar, Berkeley; Scott Lynn, Walnut Creek, all of Calif.

[73] Assignee: The Regents of the University of California, Berkeley, Calif.

[ 22] Filed: Nov. 22, 1971 [21] Appl. No.: 201,129

[52] US. Cl 62/66, 55/240, 55/268, 34/75 [51] Int. Cl B0ld 53/06 [58] Field of Search 34/5, 75; 55/82,

[56] References Cited UNITED STATES PATENTS 3,253,350 5/1966 Kals 34/75 3,541,805 11/1970 Kumar et al. 62/66 Primary Examiner-William E. Wayner Assistant Examiner-william E. Tapolcai, Jr. Attorney-Stanley Bialos [57] ABSTRACT A flowing film of a composition of water and a solute over a refrigerated surface, is at such solute concentration and freezing temperature as to de-sublimate or condense the water vapor as ice which is carried away by the film, and subsequently separated from the composition by filtering or centrifuging. A terminal portion of the refrigerated surface is shielded to prevent access of water vapor thereto to effect removal as ice of water that may be absorbed in the film from the vapor, thereby maintaining the film concentration substantially uniform. A salt solution or brine provides an advantageous solute.

12 Claims, 2 Drawing Figures ICE RFMOV i Patented Sept. 18,- 1973 CHILLER* (opflondl) TANK FLOW

FLOWMETER CONTROL VALVE ICE REMOVAL- TO VACUUM PUMP SAMPLE COLLECTOR SFPARATOR BACKGROUND OF THE INVENTION Removal of water vapor is required in many continuous processes in industry. In environments where the vapor removal is effected under freezing conditions, such as in conventional freeze-drying processes where large quantities of water vapor are evolved and must be removed, the vapor deposits as ice on solid refrigerated surfaces; and the frost or ice must be periodically removed from the refrigerated surfaces. This is disadvantageous in continuous systems. Absorption methods have been proposed wherein the water vapor is absorbed in cooled glycol, strong salt solution or other absorbing liquid desiccant. In such a process, the liquid desiccant has to be regenerated by boiling out the absorbed water. The complexity of regeneration equipment and the heat load of the regenerator militate against this type of process.

Another type of process is disclosed in the US. Pat. Nos. 2,613,513, to Shields, dated Oct. 14, 1952 and 3,541,805, to Kumar, King [I] and Morgan Jr. (two of whom are applicants hereof) dated Nov. 24, 1970, wherein a film or sheet of a water-immiscible organic liquid is caused to flow over a cold or refrigerated surface (flowing-film type). The water vapor condenses onto the surface of the organic liquid film, as ice, and is carried out with it.

The Kumar et al patent explains in more detail the problems encountered in prior types of systems for the removal of water vapor.

SUMMARY AND OBJECTS The invention hereof is of the flowing-film type disclosed in the aforementioned Kumar et al. patent insofar as the physical steps of condensing ice on a flowing film and subsequently removing the ice are concerned. However, it has been found pursuant to this invention that instead of a special type of organic liquid for effecting formation of the flowing film, water containing a solute may be employed provided the composition of the solution is maintained so that the film has a low freezing point and is in liquid condition under the environment in which the vapor deposits as ice on the film.

As is well known, a solute in water depresses or lowers the freezing point of the solution, the higher the concentration of the solute the lower the freezing point or in other words vapor pressure of the solution, up to a concentration at which a eutectic mixture of water and solute is formed. At the freezing point of such eutectic mixture composition, it becomes solid and will not flow at lower temperatures. Thus, the percentage of solute in the water should be less than the percentage which will form an eutectic mixture in order for the solution to remain a liquid near the freezing point of said eutectic mixture. Lower solute percentages may be employed in the film-forming composition hereof, depending upon the temperature of the environment in which the flowing film is utilized. However, in any event, it is desirable that the solution have a freezing point of lower than about -l0.0C in order to provide rapid condensation of water into ice.

Various salts which will form brines and non-salts may be employed as the solute; all of which will depress or lower the freezing point of the water solution thereof depending on their respective concentrations. For any particular vapor freezing temperature below about -l0.0C, one can readily tell what concentration or range of concentrations of the solute is required from the equilibrium phase diagram. Thus, for example, in the freeze-drying of coffee wherein it is desirable to have an operating temperature of the order of about 50.0C, the concentration of any particular solute in the water, to provide an operating range around that temperature and above freezing point of the eutectic mixture thereof, can be readily determined from such equilibrium phase diagram. Particular examples will be described hereinafter.

The flow rate of the solution should be such that under the circumstances at which the film is flowed over the refrigerated surface, its flow is non-turbulent or in other words laminar so as to provide smooth and efficient condensing of ice onto the surface of the refrigerated film and carry the ice away without excessive dissolution of the ice or deposition of ice on the refrigerated surface. Also, to effect efficient heat transfer from the refrigerated surface to the film so that ice may readily form on the exposed surface of the film, and yet not have ice deposit directly onto the refrigerated surface itself, the flow rate should be such that the film thickness is maintained between about 0.25 to 1.0 millimeter (mm). The viscosity of the solution should not be much greater than about 200 centipoise (cp) because much higher viscosities adversely affect heat transfer from the refrigerated surface to the water vapor condensing on the film as ice; and moreover much higher viscosities impede pumpability for effecting'laminar flow.

The invention can be employed in any operation where it is desirable to remove water vapor by condensation of the vapor as ice, such as in the freeze drying of pharmaceutical preparations and food products. For the freeze drying of food products in which the invention has particular applicability, the solute should be non-toxic, and inexpensive compared to organic liquids of the type disclosed in the aforementioned Kumar et al. patent. Alkali metal and alkaline earth metal salts, desirably halides, having low eutectic mixture freezing points and ready solubility in water are advantageous because of their relatively low cost and non-toxicity. Desirably, calcium chloride having a eutectic mixture freezing point of about 5l.0C and lithium chloride having a eutectic mixture freezing point of about --75.0C are most advantageous. Other suitable salts include magnesium chloride and sodium chloride. Other suitable solutes, among others, are glycerine, sucrose, corn syrup solids, and sulfuric acid.

, As the ice carrier film flows over the refrigerated surface and the water vapor condenses as ice thereon, some vapor will become dissolved in the film as water liquid thus decreasing the concentration of the solute in the film and hence increasing the freezing point of the solution. It may, therefore, be necessary from time to time to add make-up solution in the system to maintain the concentration of solute and provide the desired freezing point for the environment.

To minimize such adjustments, the invention hereof provides for shielding off a downstream terminal section of the refrigerated surface to preclude substantial access of water vapor thereto. This is obtained by a shield which has minimum clearance adjacent to the refrigerated surface but sufficient to allow flow of solution and ice therethrough. Such shielded downstream terminal portion of the refrigerated surface should be sufficiently long to allow excess water liquid from the vapor absorbed in the film above the shield to become frozen as ice thus restoring solute concentration. The latter ice flows out with the ice condensed on the film, and is removed therewith. The solution composition will then be at substantially the right concentration for the next cycle in the continuous operation.

From the preceding, it is seen that the invention has as its objects, among others, the provision of an improved vapor condensation system and method of the flowing-film type wherein the film is a solution of a so]- ute in water maintained at a freezing temperature range to condense ice thereon while the film remains liquid to flow ice therewith, and the solute is non-toxic and of relatively low cost; and the provision of an improved apparatus and method for preventing access of water vapor to a downstream terminal portion of the refrigerated surface to maintain the concentration of the solute substantially uniform for successive cycles of operation.

Other objects will become apparent from the following more detailed description, and accompanying drawings in which:

DETAILED DESCRIPTION FIG. 1 is a schematic view of a form of apparatus hereof; and

FIG. 2 is a horizontal section taken in a plane indicated by the line 2 2 in FIG. 1.

The apparatus except for the aforementioned shield, is of the general type disclosed in the aforementioned Kumar et al. patent. It comprises a cylindrical upright chamber 2 having inlet 3 communicating with a water vapor source, and outlet connection 4 to a vacuum pump for maintaining the environment in chamber 2 under a desired vacuum. Extending into chamber 2 is a refrigerated tube 6 closed at its bottom and containing a refrigerant flow tube 7 open at its lower end and connected at its upper end at 8 to an inlet source of any suitable refrigerant. The refrigerant flows out of refrigerated tube 6 through connection 9 communicating at its upper end with the space between refrigerant flow tube 7 and refrigerated tube 6.

Thus, the exterior surface I] of refrigerated tube 6 provides a refrigerated surface for the film-forming solution which is continuously pumped into a nozzle 12 having annular discharge rim 13 about refrigerated tube 6 at its upper end to cause flow of an annular film or sheet of the solution hereof downwardly along the entire length of refrigerated tube 6 in contact with the refrigerated outer surface 11. Ice condenses on the film, and is carried out with the film through the discharge outlet 14 at the bottom of chamber 2. A terminal portion of refrigerated surface 11 is desirably shielded from substantial surface 11 is desirably shielded from substantial access thereto of water vapor, by a plate 16 in sealed contact with the inner surface of chamber 2 but with clearance 17 about refrigerated surface 11 which will be described later in greater detail. Any suitable refrigerant, such as FREON-l2," methyl alcohol, or liquid ammonia may be employed, which by evaporation or convection cools the refrigerated surface 11 to a desired temperature as it is continuously pumped through tube 6.

From outlet 14, the film which now becomes a slurry of the solute composition and ice is discharged by means of pump 18 into a sample collector 19 which need not be a part of the actual apparatus but was employed for test purposes to measure the amount of ice obtained; the sample collector being refrigerated as indicated by R. The pump discharges the slurry of ice and solution into a separator 21 which effects separation of the ice by any suitable means such as centrifuging or filtration. From separator 21, the solution flows into a storage tank 22 containing the film forming composition of water and solute, and which is refrigerated as indicated at R.

Make up solution of the film forming solution may be supplied to tank 22 as required, to maintain the concentration of solute therein. From tank 22, the solution is pumped through flow control valve 23, a conventional flow meter 24, and through connection 26 to nozzle 12. If desired as optional equipment, a refrigerated chiller 27 may be connected after the flow meter to compensate for heat gains which might be caused by leaks.

As was previously indicated, a wide variety of solutes may be employed to provide the aqueous solution for the flowing film. The concentration of the solute in the water should be such as to provide a freezing point of the solution of below about l0.0C to insure quick freezing of the water vapor; and to insure that water vapor will condense as ice and that the solution will remain liquid so as to flow freely over refrigerated surface II, it should be below the eutectic mixture concentration thereof. The following table lists typical suitable solutes, the weight percent thereof in water which will provide a freezing point of about l0.0C, the freezing point of a eutectic mixture thereof (EM-FP), and the weight percent of solute in the eutectic mixture (EM):

Wt.% at Wt.% in

- l0.0C EM-FP-"C EM Calcium chloride 12.0 -S1.0 29.3 Lithium chloride 10.0 75.0 25.0 Ma nesium chloride 11.0 -30.3 21.0 Sodium chloride 13.0 21.0 23.0 Sucrose 58.0 14.0 63.0 Glycerine (Glycerol) 30.0 32.0 60.0 Corn Syrup Solids 46.0 25.0 70.0

As was previously indicated, a desirable solute concentration may be readily determined from the equilibrium phase diagram to provide a proper freezing point for the environment in which the ice condensing film is to be employed. As an illustration, the freeze-drying of meat is usually carried out with a condenser temperature of about 35.0C. Consequently, magnesium chloride, sodium chloride, sucrose, glycerin and corn syrup solids would not be suitable because their eutectic-mixture freezing points are not sufficiently low to allow the solution to remain a free flowing film under a 35.0C environment. However, these solutes can be used in environments where the temperature is above their eutecticmixture freezing points.

Calcium chloride and lithium chloride have very low eutectic mixture freezing points of 5l.0C and 75.0C, respectively, rendering them suitable for such use as freeze-drying of meat. The freeze-drying of coffee vapor condensation is at about ,50.0C. Therefore for such purpose, calcium chloride at a percentage composition slightly less than about 29.3 weight percent is marginally suitable, and lithium chloride at a weight percent about 25.0 is quite suitable. Because of their low eutectic mixture freezing points and high so]- ubility in water, calcium chloride and lithium chloride are suitable for most purposes, and are most' advantageous solutes.

The solution is pumped at such rate as to provide non-turbulent or laminar flow over refrigerated surface 11, and to provide a film thickness of about 0.25 to 1.0 mm for efiicient heat transfer. For any particular concentration of solute in water, the rate of flow can be readily determined to provide the laminar flow and desired film thickness.

Refering again to shield 16, a small amount of the ice which condenses on the film flowing over refrigerated surface 11 and also a small amount of water vapor will become absorbed as water in the surface layers of the film, thus gradually decreasing the concentration of the solute with consequent raising of the freezing point. This can be compensated for by adjusting the solute concentration through addition of solution or solute from time to time, as required. However, to minimize the frequency of adjustments, shield 16 provides a means for maintaining the concentration substantially uniform for relatively long periods by continuously restoring the concentration. Its clearance about refrigerated surface 11 is merely sufficient to permit flow of the film with a sheet of ice thereover, past the shield, and at the same time prevent or block off substantially, flow of water vapor along the terminal portion of refrigerated surface 11 below the shield. A clearance of about 5 times the film thickness is suitable, namely about 1.25 to 5.0 mm. Shield 16 may be of any suitable material, preferably metal.

In such substantially vapor free section below shield 16, the slight amount of excess water which has be come absorbed in the surface layers of the film above shield 16 will freeze as ice in the film thus increasing and restoring solute concentration. In this connection, the terminal portion of refrigerated surface 11 is at least of such length, which can be readily determined for any operating conditions, to effect substantially equalization of temperature of the outer surface of the film and the temperature of the refrigerated surface, thus maintaining the solute concentration substantially uniform for the next cycle of operation after the composition has been freed of ice and pumped back into nozzle 12. Excess length is immaterial while if the length is less, then concentration adjustments will be more frequent.

The desired length can be determined more closely by making the shield adjustable lengthwise of refrigerated tube 6, and positioning it where it is found that the solute concentration is substantially uniform. The following formula may be employed for obtaining a close approximation for the length of such terminal portion:

wherein:

L length in feet of terminal portion v velocity of flow of film in feet per second t thickness of film in feet d thermal diffusivity of solution in square feet per second Calcium chloride is an advantageous solute, particularly for the freeze-drying of food products, because of its low cost, high solubility in water and non-toxicity. A desirable concentration of the calcium chloride in water to form a film of the character indicated is slightly below but near the eutectic mixtur'e freezing point namely about 29 weight percent. With such concentration the following example illustrates a particular embodiment of the invention under test conditions in which the shield 16 was not employed, as it is not essential to the principle of the invention although desirable.

Tube 6 was about 1.75 feet in length and 1.0 inch outside diameter having a wall thickness of about 0.03 inch and was corrosion resistant nickel plated copper tubing to provide high heat transfer. The refrigerant introduced continuously into tube 6 was methyl alcohol which produced a temperature of about -52.6C. Pure water vapor was continuously introduced into chamber 2 which was a glass tube for observation purposes, of about 3.0 inches outside diameter and about 2.5 feet in length.

The water vapor continuously introduced into chamber 2 was at a temperature of about 250 C. and the vacuum for effecting the continuous flow of vapor was about 0.3 mm of mercury. The rate of flow of the calcium chloride brine was about 55 pounds per hour at a temperature of about 45 C. and the brine had a concentration of 28.9 weight percent, providing a film thickness of about 0.6 mm, and a viscosity of about 16 cp. The temperature of the film surface at the lower end of refrigerated surface 11 was about 39.3 C.

The amount of water vapor introduced into chamber 2 under the above conditions was about 0.503 pounds per hour; and in checking the amount of ice obtained in the sample collector 19, it was found to be about 0.43 pounds per hour which was about an percent recovery from the water vapor. As previously noted, make-up brine was added from time to time to adjust the concentration.

Although one refrigerated tube was employed in the example, the apparatus may comprise a plurality of such tubes in a chamber in which the water vapor is condensed as ice, to thus provide refrigerated surfaces over which a plurality of streams of the film forming solution can flow, with each of the tubes extending through a common shield of the character described which is in sealed contact at its periphery with the inner surface of the chamber.

We claim:

1. In the method of condensing and removing water vapor wherein the vapor is in contact with a film of liquid flowing over a refrigerated surface which maintains the liquid film at a temperature below the freezing point of water whereby the vapor condenses as ice on said film which is carried away by the film, and said ice is subsequently separated from the film, the improvement comprising utilizing as the composition for the film, a solution of water and a solute which lowers the freezing point of the water and which is of such concentration in the water as determined by the equilibrium phase diagram of said solution to condense the water vapor as ice on the film, the concentration of the solute being less than that of the eutectic mixture of said solution to maintain said film in the liquid state.

2. The method of claim 1 wherein the solute concentration is in an amount sufficient to impart a freezing point to the composition of below about l0.0C, and said concentration is adjusted at intervals by addition of said solute thereto to compensate for dilution.

3. The method of claim 1 wherein said solute is a water-soluble salt.

4. The method of claim 1 wherein said solute is glycerol.

5. The method of claim 1 wherein said solute is sucrose, glucose, or corn syrup solids.

6. The method of claim 3 wherein said salt is an alkaline earth or alkali metal halide. 4 1

7. The method of claim 3 wherein said salt is calcium chloride in a concentration of about 12.0 to about 29.0 weight percent to impart a freezing point to said solution of about l0.0C, to 50.0C.

8. The method of claim 3 wherein said salt is lithium chloride in a concentration of about 10.0 to about 24.9 weight percent to impart a freezing point to said solution of about l0.0C, to 74.0C.

9. In the method of claim 1 wherein said refrigerated surface is within an enclosed chamber through which the water vapor flows and the film is caused to flow downwardly over said surface, the improvement further comprising providing a shield downstream of said refrigerated surface with clearance adjacent said refrigerated surface sufficient to allow flow therethrough of the film and ice carried thereby but which prevents substantial flow of water vapor into the section of said chamber below said shield to thereby cause the section of the refrigerated surface below the shield to freeze water absorbed by said film from the water vapor above said shield.

10. The method of claim 9 wherein the section of said refrigerated surface below the shield is at least of such length as to effect substantially equalization of the temperature of the outer surface of the film and the temperature of said refrigerated surface to substantially restore the solute concentration.

11. Apparatus for condensing and removing water vapor comprising an upright refrigerated surface within an enclosed chamber through which water vapor flows, means to effect flow of a film of a liquid solution comprising water and a solute downwardly over said refrigerated surface, said solute lowering the freezing point of the water whereby the vapor condenses as ice on said film which is carried away from said film; and a shield downstream of said refrigerated surface having clearance adjacent said refrigerated surface sufficient to allow flow therethrough of the film and ice carried thereby but which prevents substantial flow of water vapor into the section of said chamber below said shield to thereby cause the section of the refrigerated surface below the shield to freeze water abosrbed by said film from the vapor above said shield.

12. The apparatus of claim 11 wherein the refrigerated surface is the outside wall of a tube, and said shield surrounds said tube.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 759, O l? Dated September 18, 1973 Inventor(s) Cary Judson King, III; Romesh Kumar; Scott Lynn It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 63, delete the entire line reading, "shielded from substantial surface ll is desirably".

Signed and sealed this 2nd day of April 1971 (SEAL) Attest:

EDWARD MELETCHER R- I I I a i c. MARSHALLDANN Attesting Officer Commissioner of Patents FORM PO-1050I10-69) USCOMM-DC 60376-P69 U.S. GOVERNMENT PRINTING OFFICE: I969 0-355-33l 

2. The method of claim 1 wherein the solute concentration is in an amount sufficient to impart a freezing point to the composition of below about -10.0*C, and said concentration is adjusted at intervals by addition of said solute thereto to compensate for dilution.
 3. The method of claim 1 wherein said solute is a water-soluble salt.
 4. The method of claim 1 wherein said solute is glycerol.
 5. The method of claim 1 wherein said solute is sucrose, glucose, or corn syrup solids.
 6. The method of claim 3 wherein said salt is an alkaline earth or alkali metal halide.
 7. The method of claim 3 wherein said salt is calcium chloride in a concentration of about 12.0 to about 29.0 weight percent to impart a freezing point to said solution of about -10.0*C, to -50.0*C.
 8. The method of claim 3 wherein said salt is lithium chloride in a concentration of about 10.0 to about 24.9 weight percent to impart a freezing point to said solution of about -10.0*C, to -74.0*C.
 9. In the method of claim 1 wherein said refrigerated surface is within an enclosed chamber through which the water vapor flows and the film is caused to flow downwardly over said surface, the improvement further comprising providing a shield downstream of said refrigerated surface with clearance adjacent said refrigerated surface sufficient to allow flow therethrough of the film and ice carried thereby but which prevents substantial flow of water vapor into the section of said chamber below said shield to thereby cause the section of the refrigerated surface below the shield to freeze water absorbed by said film from the water vapor above said shield.
 10. The method of claim 9 wherein the section of said refrigerated surface below the shield is at least of such length as to effect substantially equalization of the temperature of the outer surface of the film and the temperature of said refrigerated surface to substantially restore the solute concentration.
 11. Apparatus for condensing and removing water vapor comprisIng an upright refrigerated surface within an enclosed chamber through which water vapor flows, means to effect flow of a film of a liquid solution comprising water and a solute downwardly over said refrigerated surface, said solute lowering the freezing point of the water whereby the vapor condenses as ice on said film which is carried away from said film; and a shield downstream of said refrigerated surface having clearance adjacent said refrigerated surface sufficient to allow flow therethrough of the film and ice carried thereby but which prevents substantial flow of water vapor into the section of said chamber below said shield to thereby cause the section of the refrigerated surface below the shield to freeze water abosrbed by said film from the vapor above said shield.
 12. The apparatus of claim 11 wherein the refrigerated surface is the outside wall of a tube, and said shield surrounds said tube. 